The present application relates to a positive electrode active material, a positive electrode, a nonaqueous electrolyte cell, and a method of preparing a positive electrode active material. More particularly, the application relates to a positive electrode active material, a positive electrode, a nonaqueous electrolyte cell, and a method of preparing a positive electrode active material by which to realize a nonaqueous electrolyte cell having high performance and showing little capacity deterioration when subjected to charging and discharging in high-temperature environments. In detail, the present application relates to a positive electrode active material including lithium-transition metal composite oxide.
Attendant on spreading of portable apparatuses such as video cameras and notebook type personal computers, in recent years, there has been an increasing demand for small-sized high-capacity secondary cells and batteries. The secondary cells in use at present include the nickel-cadmium cell and the nickel-hydrogen cell, which use an alkaline electrolyte. These secondary cells, however, are disadvantageous in that the cell voltage is as low as about 1.2 V and it is difficult to enhance energy density. In view of this, nowadays, the lithium ion secondary cell which is higher in voltage than other cell systems and high in energy density has come into widespread use.
Due to its higher charged voltage as compared with other cell systems, however, the lithium ion secondary cell has a problem in that when used in such mode as to be left for a long time in a charged state, its capacity would be deteriorated and its useful life would be shortened. In addition, when the lithium ion secondary cell is used under high-temperature environmental conditions, a rise in internal resistance would proceed, making it very difficult to secure a sufficient capacity. Solution to these problems is keenly requested.
LiCoO2, LiNiO2 and other lithium-transition metal composite oxide particles are widely used as positive electrode active materials of Lithium-ion secondary batteries. Lately, various arts to improve the state of the particle surfaces to obtain better properties of the lithium-transition metal composite oxide particles by forming coating layer at the particle surfaces or diffusing some material from the particle surfaces have been proposed.
For example, in Japanese Patent No. 3197763 (hereinafter referred to as Patent Document 1), there is shown a method in which a metallic salt or hydroxide is added to a positive electrode. Besides, Japanese Patent Laid-open No. Hei 5-47383 (hereinafter referred to as Patent Document 2) shows a technology wherein a surface of lithium cobaltate (LiCoO2) is coated with phosphorus (P). Japanese Patent No. 3172388 (hereinafter referred to as Patent Document 3) and Japanese Patent No. 3691279 (herein after referred to as Patent Document 4) shows a method in which a positive electrode active material or a surface of a positive electrode is coated with a metallic oxide.
Japanese Patent Laid-open No. Hei 7-235292 (hereinafter referred to as Patent Document 5), Japanese Patent Laid-open No. 2000-149950 (hereinafter referred to as Patent Document 6), Japanese Patent Laid-open No. 2000-156227 (hereinafter referred to as Patent Document 7), Japanese Patent Laid-open No. 2000-164214 (hereinafter referred to as Patent Document 8), Japanese Patent Laid-open No. 2000-195517 (hereinafter referred to as Patent Document 9), Japanese Patent Laid-open No. 2001-196063 (hereinafter referred to as Patent Document 10), Japanese Patent Laid-open No. 2002-231227 (hereinafter referred to as Patent Document 11), and the like show a method in which surfaces of particles are uniformly coated with a lithium-transition metal composite oxide and a method in which the composite oxide is diffused from the surfaces of particles. In addition, Japanese Patent Laid-open No. 2001-256979 (hereinafter referred to as Patent Document 12) shows a positive electrode active material in which lumps of a metallic oxide are deposited onto a metallic oxide layer. Japanese Patent Laid-open No. 2002-164053 (hereinafter referred to as Patent Document 13) shows a positive electrode active material in which at least one surface treatment layer containing at least two coating elements is formed on surfaces of cores which contain a lithium compound.
Japanese Patent No. 3157413 (hereinafter referred to as Patent Document 14) discloses a positive electrode active material in which a coating including a metallic fluoride is provided on surfaces of particles, and Japanese Patent No. 3141858 (hereinafter referred to as Patent Document 15) shows a coating which includes a crystalline metallic fluoride. In addition, Japanese Patent Laid-open No. 2003-221235 describes that the XPS (X-ray Photoelectron Spectroscopy) energy of fluorine on the surfaces of particles is specified. When the present inventors prepared the positive electrode active material by the method of mixing a metallic fluoride and heat-treating the mixture according to the disclosure, a practical effect on high-temperature preservability was observed, but the effect was limited to the effect on the surfaces of the particles and was insufficient on the basis of practical use performance. Further, U.S. Pat. No. 7,364,793 (hereinafter referred to as Patent Document 16) discloses a material obtained by a method in which a compound having high affinity for lithium and capable of supplying cations is brought into reaction with a lithium-transition metal composite oxide.